This invention relates to the recovery of constituents from subterranean formations, and more particularly to an in situ method of recovery that is particularly effective for the protection of shale oil from oil shale in an in situ retort. The term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a formation comprising marlstone deposit containing an organic material called "kerogen" which upon heating decomposes to produce carbonaceous liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid product is called "shale oil."
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972, to Donald E. Garrett, assigned to the assignee of this application, and incorporated herein by reference. This patent describes in situ recovery of liquid and gaseous carbonaceous materials from a subterranean formation containing oil shale by mining out a portion of the subterranean formation. Then explosive charges dispersed through a portion of the remaining formation are detonated to fragment and expand the portion of the remaining formation to form a stationary, fragmented, permeable mass of formation particles containing oil shale, referred to herein as an insitu oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of an oxygen supplying combustion zone feed into the retort on the trailing size of the combustion zone to advance the combustion zone through the fragmented mass. In the combustion zone oxygen in the gaseous feed mixture is depleted by reaction with hot carbonaceous materials to produce heat and combustion gas. By the continued introduction of the oxygen supplying feed into the combustion zone, the combustion zone is advanced through the fragmented mass. The effluent gas from the combustion zone passes through the retort on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products and a residue of solid carbonaceous material. The resulting liquid and gaseous products pass to the bottom of the retort for collection.
It is desirable that the retort contain a reasonably uniformly fragmented, reasonably uniformly permeable mass of formation particles having a reasonably uniformly distributed void volume or void fraction so gases can flow uniformly through the retort and result in maximum conversion of kerogen to shale oil. A uniformly distributed void fraction in the direction perpendicular to the direction of advancement of the combustion zone is important to avoid channeling of gas flow in the retort. The creation of a mass of particles of uniform void volume distribution prevents the formation of over-sized voids or channels which hinder total recovery of shale oil and also provides a uniform pressure drop through the entire mass of particles. In preparation for the described retorting process, it is important that the formation be fragmented and displaced, rather than simply fractured, in order to create high permeability; otherwise, too much pressure differential is required to pass gas through the retort. It is important that the retort contain a substantially uniformly fragmented mass of particles so uniform conversion of kerogen to liquid and gaseous products occurs during retorting. A wide distribution of particle size can adversely affect the efficiency of retorting because small particles can be completely retorted long before completion of retorting the core of large particles.
It has been proposed that oil shale be prepared for in situ recovery by first undercutting a portion of the formation to remove from about 5% to about 25% of the total volume of the in situ retort being formed. The overlying formation is then expanded by detonating explosives placed in the formation to fill the void created by the undercut.
The general art of blasting rock formations is discussed in The Blaster's Handbook, 15th Edition, published by E.I. DuPont de Nemours & Company, Wilmington, Del.
One method of explosive expansion is the so-called "V-cut" method, described at pp. 246-7 of The Blasters' Handbook, in which explosive charges are arranged within the formation and detonated in sequence so the formation is expanded in concentric sequential steps moving radially outwardly and upwardly within the formation generating a conical free face which propagates upwardly through the formation in accordance with the time delays between the explosive charges. A free face is the exposed surface of a mass of rock such as a surface in the vicinity of a shothole at which rock is free to move under the force of an explosion. A purpose of the V-cut method of expansion is to produce particles of relatively small size; but it has the disadvantage of tending to create a radially nonuniform void volume distribution throughout the expanded mass.
Rather than using the V-cut method of expansion, it has been proposed to use a plurality of concentrated charges uniformly distributed throughout the formation to be expanded to produce a uniformly fragmented mass of formation particles. U.S. Pat. No. 3,434,757 issued to Prats teaches sequential detonation of a series of explosive in oil shale to form a permeable zone in the oil shale. However, it is both time consuming and expensive to place a large number of explosive charges throughout the formation.
Another method for preparing formations for in situ recovery is described in U.S. Pat. No. 4,043,597, assigned to the assignee of this invention, and incorporated herein by this reference. According to this patent, two voids vertically spaced apart from each other are excavated in the subterranean formation. This leaves a zone of unfragmented formation between the voids. Vertical blasting holes are formed in the intervening zone. Explosive is placed in the blasting holes and detonated to expand formation in the intervening zone toward both voids.